[IMPALA-5573] Support decimal codegen in text scanner

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: Impala 2.10.0
• Fix Version/s: Impala 2.10.0
• Component/s: Backend
• Labels: codegen
• Target Version: Product Backlog
• Epic Color: ghx-label-2
• Assignee: Tianyi Wang
• Reporter: Tim Armstrong
• Created: 23/Jun/17 18:29
• Updated: 23/Aug/17 17:26
• Resolved: 23/Aug/17 17:26

Description

Codegen is disabled when scanning text tables with decimal columns. The message is "Decimal not yet supported for codegen."

The supported types use cross-compiled conversion functions in TextConverter::CodegenWriteSlot():

    IRFunction::Type parse_fn_enum;
    Function* parse_fn = NULL;
    switch (slot_desc->type().type) {
      case TYPE_BOOLEAN:
        parse_fn_enum = IRFunction::STRING_TO_BOOL;
        break;
      case TYPE_TINYINT:
        parse_fn_enum = IRFunction::STRING_TO_INT8;
        break;
      case TYPE_SMALLINT:
        parse_fn_enum = IRFunction::STRING_TO_INT16;
        break;
      case TYPE_INT:
        parse_fn_enum = IRFunction::STRING_TO_INT32;
        break;
      case TYPE_BIGINT:
        parse_fn_enum = IRFunction::STRING_TO_INT64;
        break;
      case TYPE_FLOAT:
        parse_fn_enum = IRFunction::STRING_TO_FLOAT;
        break;
      case TYPE_DOUBLE:
        parse_fn_enum = IRFunction::STRING_TO_DOUBLE;
        break;
      default:
        DCHECK(false);
        return NULL;
    }

Decimal is a bit different to these functions because it accepts precision and scale parameters and the parsing functions are templated. However I think in principle the same approach can be used as for the other types.

Note: StringToDecimal() is a huge inline function and we risk run into codegen-time problems if we're scanning many decimal columns. We should experiment to see if this is a problem and if so try to mitigate it, e.g. by not moving the function out of the header.